1. Technological Field
The present invention relates generally to the measurement of optical signals, and more specifically to a measurement apparatus and method for measuring a high-frequency optical signal under test.
2. Background Art
In recent years, optical technology has come to be used in many different fields, including measurement, communications, recording, machining, etc. In many of these fields, higher speeds have become more and more important, and high-speed optical technology has become particularly important in the field of telecommunications. The information signals with which optical signals used in these fields are modulated have reached double-digit gigabytes per second (Gbps) levels and will increase in speed in the future.
Thus, there is a growing need to provide an apparatus and method for measuring optical signals. Optical sampling is particularly suited to measurement of high-frequency optical signals, which are optical carrier signals modulated with information signals having frequencies in a wide range from several gigahertz (GHz) to several dozen terahertz (THz). Moreover, such optical sampling measurement apparatus is required to be capable of accommodating optical signals whose carrier frequencies are substantially higher than that of the modulation frequencies.
Further, for a high-frequency optical signal measurement apparatus to be practical, the apparatus requires extra capabilities, such as being able to deal flexibly, easily, and quickly with new measurement requirements while having the ability to deal with high modulation frequencies.
A search for such apparatus and methods has been long ongoing, but has long eluded those skilled in the art.
The present invention provides a measurement apparatus and method for an optical signal under test. The apparatus includes a closed-loop optical path, an optical mixer in the closed-loop optical path and a photodetector. The optical signal under test and sampling light having a wavelength different from that of the optical signal under test are circulated in the closed-loop optical path. Sum/difference frequency light is generated every time the sampling light passes through the optical mixer. The sum/difference frequency light is detected by the photodetector, which provides a signal representative of the waveform of the optical signal under test.
In the method, the optical signal under test and sampling light having a wavelength different from that of the optical signal under test are circulated at different speeds. Sum/difference frequency light is generated for each circulation of the optical signal under test and the sampling light. The sum/difference frequency light is detected and a detector signal representative of the waveform of the optical signal under test is provided.
The apparatus and method are capable of optically sampling optical signals under test modulated with information signals having frequencies in a wide range from several gigahertz (GHz) to several dozen terahertz (THz).
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawing.